Jayph,
I assume you're looking at the TAC338 since it is designed for the 338's and larger cartridges.
I own a Stiller TAC300 and it is a good action, the bolt is made by PT&G although I think I heard him say he makes them in house now.
The Bolt fit is not as tight as on some other customs because it's designed for tactical and hunting applications where a little more room is desired, 4 to 6 thousands. This is actually good if you are planning on painting your action (Duracoat or Cerakote).
Good luck with your project.
T